The hydrogen economy is a proposed method of deriving the energy needed for motive power (cars, boats, airplanes), buildings or portable electronics, by reacting hydrogen (H₂) with oxygen, the hydrogen having been generated by a number of possible methods, including the electrolysis of water. If the energy used to split the water were obtained from renewable or nuclear power sources, and not from burning carbon-based fossil fuels, a hydrogen economy would greatly reduce the emission of carbon dioxide and therefore play a major role in tackling global warming. Countries without oil, but with renewable energy resources, could use a combination of renewable energy and hydrogen instead of fuels derived from petroleum, which are becoming scarcer, to achieve energy independence.

In the context of a hydrogen economy, hydrogen is an energy storage medium, not a primary energy source (see nuclear fusion for an entirely separate discussion of using hydrogen isotopes as an atomic energy source). Nevertheless, controversy over the usefulness of a hydrogen economy has been confused by issues of energy sourcing, including fossil fuel use, global warming, and sustainable energy generation. These are all separate issues, although the hydrogen economy affects them all (see below).

Proponents of a world-scale hydrogen economy show that hydrogen can be an environmentally cleaner source of energy to end-users, particularly in transportation applications, without release of pollutants (such as particulate matter) or greenhouse gases at the point of end use. Analyses have concluded that "most of the hydrogen supply chain pathways would release significantly less carbon dioxide into the atmosphere than would gasoline used in hybrid electric vehicles" and that significant reductions in carbon dioxide emissions would be possible if carbon capture or carbon sequestration methods were utilized at the site of energy or hydrogen production.

Critics of a hydrogen economy argue that for many planned applications of hydrogen, direct distribution and use of energy in the form of electricity, or alternate means of storage such as chemical batteries, fuel plus fuel cells, or production of liquid synthetic fuels from CO₂ (see methanol economy), might accomplish many of the same net goals of a hydrogen economy while requiring only a small fraction of the investment in new infrastructure. Hydrogen has been called the least efficient and most expensive possible replacement for gasoline (petrol) in terms of reducing greenhouse gases. A comprehensive study of hydrogen in transportation applications has found that "there are major hurdles on the path to achieving the vision of the hydrogen economy; the path will not be simple or straightforward".